1. Field of the Invention
The invention relates to a vacuum casting apparatus that performs casting through decompression of a cavity.
2. Description of Related Art
Vacuum casting apparatuses for casting metal products and resin products (e.g., vacuum casting die-cast apparatuses) are known which are adapted to decompress the cavity in the mold before casting in order to minimize the possibility of gas bubbles being created in the molding. In some of such vacuum casting apparatuses, an ejector pin is provided which is used to release the molding from the mold. Such an ejector pin is slidably arranged in a pinhole formed in the mold.
As a conventional vacuum casting apparatus, Japanese Patent Application Publication No. 2006-068814 describes a vacuum casting apparatus having a pinhole that is formed, for reducing the resistance against sliding of the ejector pin when ejecting the molding, to have a small diameter portion that is located near the cavity of the mold and is almost equal in diameter to the ejector pin and a large diameter portion that is distant from the cavity and is larger in diameter than the ejector pin.
Meanwhile, it is often the case that a release agent is sprayed to the inner face of the cavity before casting, in order for the molding to be easily released from the mold. For vacuum casting apparatuses having an ejector pin, there is a possibility that the release agent enter the gap between the ejector pin and the pinhole during spraying of the release agent.
If the vacuum casting apparatus has the small diameter portion and the large diameter portion, such as those described above, in the pinhole, the release agent that has entered the gap between the ejector pin and the pinhole accumulates in the large diameter portion of the pinhole. FIG. 7 illustrates an example of such a state. Referring to FIG. 7, as a cavity 50 is decompressed, a release agent M accumulating in a large diameter portion 53 of a pinhole 52 in which an ejector pin 51 is arranged is sucked into the cavity 50 due to the flow of air drawn into the cavity 50 (refer to the broken line arrows in FIG. 7). As such, an excessive amount of the release agent M is brought to near the opening of the pinhole 52 in the cavity 50, and the water contained in the release agent M is taken into the molten material, creating gas pores in the molding.
The release agent may be prevented from entering the gap between the ejector pin and the pinhole, if the release agent is sprayed while sending air into the gap so as to be ejected from the gap, as described in Japanese Patent Application Publication No. 2001-071106. In such a case, however, a pump for feeding compressed air, or the like, is required, making the mold structure and casting control complicated.